Eheim, hands down, makes a better filter than SunSun. The SunSun advantage is its surprising functionality and dependability, *given its low price*. If you're interested in a top quality filter, look elsewhere. If you're looking for something cheap that works pretty well and will last, SunSun is a good bet.
As far as flow rates, in general, go, not a filter on the market is "honest". The circumstances necessary to achieve a published flow rate are difficult, or sometimes impossible, to mimic, and certainly not possible when the filter is filled with media and placed under a tank. However, the published Eheim flow rates are more honest than most.